(a) Field of the Invention
The present invention relates to a molding material for IC parts which is excellent in heat resistance represented by heat distortion temperature, moldability represented by a melt flow index and electroconductivity represented by surface specific resistance of molded articles thereof
(b) Description of the Related Art
In electronic instruments, the mounting density of parts tends to increase, and in order to heighten the mounting density, IC are mounted on through-hole substrates and the like. However, if the IC are wet at the time of the mounting thereof by flow solder, water vapor is generated in the IC during heating, with the result that flakes or cracks appear thereon and the IC are broken. For this reason, it is necessary that prior to the mounting, the IC are dried at a temperature of 100.degree. C. or more to remove moisture therefrom.
Heretofore, in the process of drying the IC, the IC must be once transferred from a plastic tray (or a magazine case plastic tray) having a surface specific resistance of 10.sup.7 .OMEGA. or less in order to prevent the breakdown of the IC by static electricity to an aluminum diecast tray, and be then dried. Afterward, the dried IC are returned to the plastic tray and are then shipped. Therefore, the complicated and burdensome process is required. The greatest reason why such a burdensome process is necessary is that the tray is made from polyvinyl chloride or polystyrene which cannot withstand heat, and is deformed at the time when the IC are dried.
Thus, for the simplification of the process and for the replacement of the expensive aluminum diecast tray, much attention is paid to a plastic tray which is resistant to a temperature of 100.degree. C. or more. On the other hand, also for wafer carriers which are used in washing silicon wafers and for racks which are used in carrying print-circuit boards in a soldering step, materials which can withstand conditions of 100.degree. C. or more are demanded. Additionally, in order to shorten a drying period of time, it is also desired to achieve drying at a higher temperature, and preferably plastic trays which are resistant to a drying temperature of 130.degree. C. or more are demanded.
As materials which can satisfy the above-mentioned requirement, polyamide resins, polyester reins and the like containing glass fiber can be contrived, but these resins are all crystalline and hence they have a high mold shrinkage ratio and are poor in dimensional stability. Furthermore, there are also amorphous resins such as polycarbonttes in which glass fiber is contained, but these resins are poor in the moldability. In addition, molded articles thereof tend to warp and have low dimensional stability. As highly heat-resistant amorphous resins, blends of polyphenylene ether resins and high-impact polystyrene resins are under investigation, but materials which can simultaneously satisfy the requirements of heat resistance and the moldability have not been obtained yet.
As a result of researches of the present inventors, it has been found that when the content of the polyphenylene ether resin is increased to 50% by weight or more with the intention of heightening heat distortion temperature up to 130.degree. C. or more, the moldability is too poor to accomplish injection molding. Even if the molding can be made, stress remains in molded articles, and an inconvenient phenomenon such as warpage occurs in a subsequent heating step. Moreover, high shear force at the time of the molding inconveniently functions to break the uniformed dispersion state of electroconductive carbon, so that the surface specific resistance of the molded articles exceeds a level of 10.sup.7 .OMEGA.. When the content of the polyphenylene ether resin is decreased to less than 50% by weight so as to improve the moldability, the heat distortion temperature is 130.degree. C. or less, and in the subsequent heating step, warpage and distortion take place similarly. Polyether sulfones, polyallylates and polyether imides can be also taken up, but these electroconductive polymers containing electroconductive carbon are not practical because of the poor mold-ability.